Polyesters such as, for example, polyethylene terephthalate, polytrimethylene terephthalate and polybutylene terephthalate, generally referred to as “polyalkylene terephthalates”, are a class of important industrial polymers. They are widely used in thermoplastic fibers, films, and molding applications.
Polyalkylene terephthalates can be produced by transesterification of a dialkyl terephthalate ester with a glycol followed by polycondensation or by direct esterification of terephthalic acid with the selected glycol followed by polycondensation. A catalyst is used to catalyze the esterification, transesterification and/or polycondensation.
Antimony, in the form of a glycol solution of antimony oxide, frequently is used as catalyst in the transesterification or esterification process. However, antimony forms insoluble antimony complexes that plug fiber spinnerets and leads in fiber spinning to frequent shutdowns to wipe spinnerets clean of precipitated antimony compounds. The antimony-based catalysts are also coming under increased environmental pressure and regulatory control, especially in food contact applications.
Organic titanates, such as tetraisopropyl and tetra n-butyl titanates, are known to be effective polycondensation catalysts for producing polyalkylene terephthalates in general, and frequently are the catalyst of choice. However, these catalysts tend to hydrolyze on contact with water, forming glycol-insoluble oligomeric species which lose catalytic activity. These organic titanates also generate a significant amount of yellow discoloration when used as polyesterification catalysts. Additionally, many organic titanate catalysts are also substantially insoluble in a polymerization mixture thereby creating non-uniform distribution of catalyst in the mixture.
U.S. Pat. No. 3,404,121 and U.S. Pat. No. 5,340,907 disclose using a combination of zinc acetate and tetraisopropyl titanate as catalysts. When metal salts such as zinc acetate are added to the reaction mass as a solid, it is difficult to control the feed rate uniformly, resulting in variation of the polymerization conditions. When added as a glycol solution, the solubility is quite low and the metal may precipitate over time. The use of a water solution is not compatible with the use of tetraisopropyl titanate because of hydrolysis. Also, water-compatible titanates, when used as polyesterification catalysts, generate significant yellow discoloration in the resultant polymer. See, for example, EP 812818 and WO 99/28033. There is, therefore, a need for a catalyst system that is compatible with water, has good catalytic activity, and produces a polymer with reduced color.
Additionally, many solutions of a titanium α-hydroxycarboxylate and a water-soluble zinc salt are not stable, in that they form a cloudy solution or a gel after only a short period of time. This is undesirable, for it may lead to poor control of the catalyst feed to the reaction zone, to an uneven product quality, or to plugging up the down stream spinnerets. Solving this potential problem by adding more water is undesirable because the added volume makes it difficult to store or ship economically. There is also a need for a water-compatible catalyst that has good catalytic activity, produces a polymer with reduced color, is environmentally friendly, does not result in plugging fiber spinnerets, is relatively concentrated, and is stable.
An advantage of the invention is that a titanium- and zinc-containing catalyst solution can be stabilized for over two weeks under accelerated storage conditions of 60° C., equivalent to much longer storage times at ambient conditions by inclusion of a carboxylic acid. Other advantages of the inventive system will become more apparent as the invention is more fully disclosed herein below.